Various electrometers are known for measuring ion currents and often take the form of a circuit that converts an analog input current signal to an output voltage signal. The output voltage signal may be an analog signal or a digital signal, and in the latter case the electrometer functions as a form of analog-to-digital converter. Electrometers have many applications such as in pressure transducers used for measuring very low gas pressures (e.g., 1E-7 Torr to 1E-2 Torr). Such pressure transducers sometimes comprise an ion gauge (for example of the "hot cathode", "cold cathode", or "partial pressure" type) for generating an ion current signal that is representative of pressure. In such a transducer, the ion gauge is housed in a low pressure gaseous environment where it emits electrons, each of which has a probability of colliding with a gas molecule, the probability being related to the density of the gas. Each collision between an electron and a gas molecule generates an ion and the ions from all the collisions are collected and output as a collector current. The ion gauge may be modelled as a current source and the collector current output of the gauge is a function of the emission current (i.e., the number of electrons emitted by the gauge) and the density of the gas, and is therefore indicative of gas pressure. The collector current is applied to the input of an electrometer which in turn generates a voltage signal representative of the measured pressure. Such a pressure transducer may also include post processing components for generating control signals to, for example, adjust a valve in response to the measured pressure.
FIG. 1 is a schematic of one prior art electrometer 10. Electrometer 10 receives two input signals, CURRENT-IN and SET/RUN, and generates a single output signal VOUT. The input signal CURRENT-IN is a current signal supplied by a current source (not shown) and may be the collector current generated by an ion gauge. The input signal SET/RUN is a control signal and the electrometer 10 operates in either a "run" mode or in a "set" mode depending on the state of the SET/RUN signal. Some form of controller (not shown) typically generates the input signal SET/RUN. When operating in the run mode, electrometer 10 generates the output voltage signal VOUT so that the value of VOUT is representative of the time integral of the input current signal CURRENT-IN. Electrometer 10 is operated in the set mode to "reset" (or "clear", or "initialize") the electrometer.
Electrometer 10 includes an operational amplifier 12 (having an inverting input, a non-inverting input and an output), two capacitors 14, 16, and two electronically controlled switches 18, 20. Capacitor 14 and switch 18 are coupled in parallel between the non-inverting input of amplifier 12 and ground. Capacitor 16 and switch 20 are coupled in parallel between the output and the inverting input of amplifier 12. The input signal CURRENT-IN is transmitted through a resistor 22 to the non-inverting input of amplifier 12, and the input signal SET/RUN is transmitted to the control inputs of switches 18, 20.
In operation, the controller (not shown) uses the input signal SET/RUN to control the state of switches 18. 20. When the switches 18,20 are closed, electrometer 10 operates in the set mode, and when the switches 18, 20 are open, electrometer 10 operates in the run mode. The controller closes switches 18,20 by setting the SET/RUN signal to a logical high value (e.g., five volts), and opens switches 18,20 by setting the SET/RUN signal to a logical low value (e.g., ground).
When the switches 18, 20 are closed, ( i.e., when electrometer 10 operates in the set mode) switch 18 couples the non-inverting input of amplifier 12 to ground and thereby clears any charge that may have built up on capacitor 14, and switch 20 couples the output of amplifier 12 to the inverting input of amplifier 12 and thereby clears any charge that may have built up on capacitor 16.
When switches 18, 20 are open, (i.e., when electrometer 10 operates in the run mode) amplifier 12 operates as a well known integrating amplifier so that the output voltage signal VOUT is related to and is a function of the capacitance of capacitors 14, 16, and the time integral of the input current signal CURRENT-IN. Since the capacitance values of capacitors 14, 16 are known and fixed, the value of the input current signal CURRENT-IN is proportional to and may be determined by monitoring the output voltage signal VOUT.
FIGS. 2A-C are graphs illustrating the idealized operation of electrometer 10. FIG. 2A is a graph of current verses time illustrating an exemplifying waveform for the input current signal CURRENT-IN. This waveform is a step function which starts at zero Amperes and at time t.sub.0 increases to a current value of I.sub.0 and then remains at the value I.sub.0 for all time. FIG. 2B is a graph of voltage verses time illustrating an exemplifying waveform for the input control signal SET/RUN. FIG. 2C is a graph of voltage verses time illustrating the resulting output voltage signal VOUT. As can be seen from FIGS. 2A-C, when the input signal SET/RUN is set to a logical high value electrometer 10 operates in the set mode and the output voltage signal VOUT is set to ground, and when the input signal SET/RUN is set to a logical low value, electrometer 10 operates in the run mode and the output voltage signal VOUT increases as a function of the time integral of the input current signal CURRENT-IN.
One problem with prior art electrometers is of the type shown in FIG. 1 is that they do not function accurately when the input current signal is very small. For example, if the input signal CURRENT-IN applied to electrometer 10 is on the order of 1 pA (Pico Ampere), leakage currents in the switches 18, 20 (as well as parasitic capacitances and resistances of the switches) significantly affect the operation of amplifier 12 so that the output signal VOUT is not a reliable indicator of the input signal CURRENT-IN. For similar reasons, prior art electrometers of the type shown in FIG. 1 also typically do not function accurately over wide dynamic ranges.
Several techniques have been attempted to improve the dynamic range and signal sensitivity of prior art electrometers of the type shown in FIG. 1. One form of prior art electrometer uses what is sometimes known as a floating point amplifier which can include multiple amplifiers having different gains and "gain switching" techniques for selecting the gain as a function of the level of the input signal so as to provide reliable operation over a much wider dynamic range of input signal. This type of electrometer is generally disadvantageous because they use relays which are expensive and which have a relatively short mean time to failure and because they use relatively large resistors which are unstable. Another form of prior art electrometer exploits the logarithmic characteristic of P-N junctions to provide logarithmic amplifiers which operate over a wide dynamic range. While this type of electrometer operates over a wide dynamic range, they are generally disadvantageous because they use more than one amplifier and are therefor expensive, and they generally have only a relatively low accuracy.
In general, such prior art electrometers have several disadvantages. They are complex circuits using large numbers of electrical components and are therefore expensive to fabricate, and further they are susceptible to a wide variety of noise sources thereby reducing their utility, particularly for very small input signal levels. Also, they often generate an analog output signal which may not conveniently be applied to a digital circuit such as a digital computer. There is therefore a need for an inexpensive electrometer that is accurate over a wide dynamic range of input signal and that may operate reliably for very small input signal levels. There is also a need for such an electrometer which generates a digital output signal.